The present invention relates to a process and an apparatus for evaluating printing quality and for regulating ink feed controls in an offset printing machine, in which the printed products as well as a reference are measured photoelectrically, by image element, and in which the comparison, by image elements, of the printed products to the reference is the basis for the determination of a quality measure and of setting values for the ink feed control elements. The present invention also relates to an offset printing machine having a device for automatically regulating ink feed control elements, which machine utilizes the apparatus of the present invention.
The evaluation of print quality and regulation of ink feed controls is usually effected by means of standardized color control strips. These control strips, printed together with the job, are evaluated densitometrically and the ink feed controls of the printing machine adjusted or set accordingly. The measurement of the color control strips may take place on the printing machine while it is running by means of so-called machine densitometers, or off-line, for example by automatic scanning densitometers, with the control circurt to the ink dosing elements either being open (quality evaluation) or closed (machine control), in both cases. A representative example of a computer-controlled printing machine with a closed control circuit is described in U.S. Pat. No. 4,200,932, among others.
In actual practice, for example for reasons of format, the use of a color control strip is frequently not possible. In such cases quality is usually evaluated visually by manual means.
U.S. Pat. No. 3,376,426 describes a multicolor printing machine regulated by means of a machine densitometer which operates without color measuring strips. In this printing machine the individual printed sheets are scanned point by point, the diffuse reflectance values are converted to densities (logarithmized) and the color densities are transformed in a nonlinear demasking operation into analytical color densities. These analytical color densities are compared directly with the analytical color densities of an "OK sheet", which were previously obtained in the same manner, and then stored. From the results of this comparison, a signal is obtained for each printing ink indicating the respective deviations of ink feed controls from the desired settings, whereby the ink dosage is then adjusted. This system, as described in U.S. Pat. No. 3,376,426, has not been found to be practical. This is probably due to the fact that secondary absorptions and the effect of overprinting are not adequately taken into account.
More recently, a system has been disclosed (for example U.S. Pat. No. 4,561,103) which makes possible the machine evaluation of printed products without using color control strips. In this system, the printed products are scanned photoelectrically over the entire image surface area on the printing machine while it is running by means of a machine densitometer, by image elements. The scanned values obtained from the individual image elements are compared (optionally, following special processing) with the similarly-processed scanned values of a reference product ("OK" sheets). From the results of the comparison, a quality decision of "good" or "poor" is reached using certain decision criteria. The decision criteria include such factors as the number of image elements differing by more than a certain tolerance from the corresponding image elements of the reference, the differences summed over selected image areas of the scanned values with respect to the corresponding scanned values of the reference, and the differences summed over certain scanning tracks of the scanned values from the corresponding values of the reference.
This system represents a certain amount of progress but is capable of improvement in several areas.
This patent is copending with related U.S. application Ser. Nos. 665,975 and 665,976, both filed Oct. 29, 1984.